A number of antennas has been developed to replace typical monopole antennas which are still widely used in motor vehicles because of their simple structure and effectiveness. However, because such antennas protrude from exterior surfaces of the vehicle, they are exposed to destructive impacts and create aerodynamic disturbances that affect performance or create noise as the vehicle travels. Moreover, retractor mechanisms for such antennas substantially increase the cost of supplying the component, and they displace the monopole from an operable, exposed position to an inoperative, retracted position where reception is obstructed by adjacent conductive parts such as engine parts, chassis parts or body panels.
One previous way to overcome such problems has been to incorporate the antenna in other body panels. For example, conductive body panels such as expanded areas of sheet metal may be employed to form slot antennas by cutting a slot into an expanded ground plane made of conductive material. Sheet metal panels of the vehicle have previously been employed to form the slot antenna. Conductor terminals are secured at locations on opposite sides of the slot to transfer the voltage signal received by the antenna. Adjusting the relative positions of the terminals on the ground plane affects the impedance of the antenna, but the ground plane is generally very large in relation to the size of the slots. Moreover, the surface area of ground plane would typically be enlarged in order to enhance the performance of the antenna. A shield of a coaxial cable may be attached to one side of the slot and the center conductor of the coaxial cable secured to the opposite side of the slot, the impedance being adjusted by moving the feed point along the length of the slot and adjusting the dimensions of the slot itself. Typically, a slot would be a half wavelength long. For example, a slot in the ground plane would be 18.75 inches long for reception of a signal at 315 megahertz.
Moreover, the directional sensitivity of the antenna is affected by the alignment of the antenna, and horizontal panels of the motor vehicles are not most advantageous for reception of higher frequency signals, for example on the order of a 315 megahertz signal used for remote keyless entry systems, or a 820-895 megahertz signal used for cellular phone systems. For example, U.S. Pat. No. 5,177,494 to Dorry et al. discloses a slot antenna arrangement in which a plurality of antennas are arranged in numerous orientations throughout the vehicle, thus substantially increasing the complexity and cost of the slot antenna system. Moreover, a ground plane aligned at a proper angle, for example a side panel or window area of the vehicle, would require a substantial surface area to be covered with a conductive material and thus tend to obscure visibility and interfere with operation of the vehicle.
Other known types of antennas have been adapted for use in the window area of motor vehicles. For example, it has been known to use the heater grid which extends across a large portion of the rear window as an AM radio signal antenna. However, such an antenna does not perform well in the FM radio frequency range and higher ranges. Accordingly, an additional antenna for reception of FM radio signals has been mounted to windows where the heated grid has been combined with developed filter circuits for reception of AM radio signals. For example, the FM antenna may be an extended conductor arranged in a zig-zag pattern across a substantial length of the rear window of the vehicle. As a result, there is very little window space left in a vehicle rear window carrying these known types of antennas for installation of additional antennas that could receive higher frequency radio signals, for example, radio signals used for remote keyless entry systems and cellular telephone systems, that would require large areas when constructed according to known techniques.